Since aliphatic polyesters such as polyglycolic acid and polylactic acid are degraded by microorganisms or enzymes present in the natural world such as soil and sea, they attract attention as biodegradable polymeric materials which impose little burden on the environment. These biodegradable aliphatic polyesters are also utilized as medical polymeric materials for surgical sutures, artificial skins, etc. because they have degradability and absorbability in vivo.
As the biodegradable aliphatic polyesters, are known polyglycolic acid (hereinafter may referred to as “PGA”) composed of a repeating unit of glycolic acid, polylactic acid (hereinafter may referred to as “PLA”) composed of a repeating unit of lactic acid, lactone-based polyesters such as poly-ε-caprolactone, polyhydroxybutyrate-based polyesters and copolymers thereof, for example, copolymers composed of a repeating unit of glycolic acid and a repeating unit of lactic acid.
Among the biodegradable aliphatic polyesters, PLA has such features that L-lactic acid which becomes a raw material is cheaply obtained from corn, root vegetables and the like by a fermentation process, the total amount of carbon dioxide emissions is small because it is derived from natural agricultural products, and it is strong in rigidity and good in transparency as the performance of the resultant poly-L-lactic acid.
On the other hand, PGA is excellent in heat resistance and mechanical strength such as tensile strength and also excellent in gas barrier properties when formed into a film or sheet in particular in addition to high degradability. Therefore, PGA is expected to be used as agricultural materials, various packaging (container) materials and medical polymeric materials, and so its new uses are developed either singly or in the form of a composite with other resin materials.
As methods for producing a product from a biodegradable aliphatic polyester, are adopted melt forming or molding methods and other methods such as extrusion, injection molding, compression molding, injection compression molding, transfer molding, cast molding, stampable molding, blow molding, stretch film forming, inflation film forming, laminate molding, calendering, foam extrusion, RIM, FRP molding, powder molding, paste molding and flow cast molding (casting).
The melt forming or molding is a forming or molding method that a composition containing the biodegradable aliphatic polyester and additives blended as needed is used as a raw material for forming or molding, and this composition is heated and melted and then formed into a predetermined shape. Pellets of the biodegradable aliphatic polyester such as PGA used as the raw material for forming or molding are those obtained by melt-extruding the biodegradable aliphatic polyester such as PGA into a strand by means of, for example, a twin-screw extruder and cutting the strand into a desired size and having an average particle size of about several millimeters.
The powder molding or paste molding is a molding method that powder or particles of the biodegradable aliphatic polyester such as PGA are applied to a mold as it is or as a dispersion liquid dispersed in a solvent and formed into a shape conforming to the mold generally under heating. As the powder or particles of the biodegradable aliphatic polyester used, are used particles prepared in predetermined size and shape according to use.
The flow cast molding (hereinafter may referred to as “casting” or “cast molding”) is a method that a solution obtained by dissolving a composition containing the biodegradable aliphatic polyester such as PGA or PLA and additives blended as needed in a solvent is cast on a drum (casting drum) the surface of which has been smoothed or a smoothed belt made of stainless steel to apply the solution thereto, and heating is then conducted as needed to evaporate or remove the solvent, thereby solidifying the biodegradable aliphatic polyester to produce a molded product in the form of often a film or sheet. The flow cast molding (casting) has such advantageous features that i) since no physical pressure is applied to the product in the form of, for example, a film, the product does not undergo orientation, and no directional property is produced in strength and optical properties, ii) thickness accuracy is extremely high, iii) the quantity of heat to be applied is small compared with the melt forming or molding method, and iv) the amount of a heat stabilizer or the like to be added can be reduced.
The biodegradable aliphatic polyester such as PGA or PLA attracts attention to the degradability, strength, etc. thereof and is expected to be used as a raw material, an additive or the like in fields of paints, coating materials, inks, toners, agricultural chemicals, medicines, cosmetics, mining, boring, etc. The biodegradable aliphatic polyester applied to these fields is used in the form of a dispersion liquid obtained by dispersing particles of the biodegradable aliphatic polyester in a solvent or in the form of a solution obtained by dissolving the biodegradable aliphatic polyester in a solvent.
In order to apply the biodegradable aliphatic polyester such as PGA or PLA to the flow cast molding (casting) or the fields of paints, coating materials and the like, the biodegradable aliphatic polyester is required to be easily dissolved in a solvent to form a uniform solution.
However, the biodegradable aliphatic polyester such as PGA or PLA is generally crystallized and high in crystallinity, so that it has been difficult to dissolve it in an organic solvent under an environment of a low temperature such as ordinary temperature. For example, Japanese Patent Application Laid-Open No. 58-206637 (Patent Literature 1) discloses that polylactide is dissolved in xylol by heating it to boiling. Japanese Patent Application Laid-Open No. 2006-45542 (Patent Literature 2) discloses that PLA is dissolved in a mixture [DBE (trademark)] of dimethyl adipate, dimethyl glutarate and dimethyl succinate at a temperature of 140° C., and that PGA is dissolved in bis(2-methoxyethyl)ether at a temperature of 150° C. In order to dissolve the biodegradable aliphatic polyester in a solvent at a high temperature, it is necessary to supply a great amount of thermal energy, and there is a possibility that hydrolysis or thermolysis may be caused because an excess of thermal history is applied to the biodegradable aliphatic polyester. Japanese Patent Application Laid-Open No. 8-311368 (Patent Literature 3) discloses that a biodegradable polymer having a number average molecular weight of at most 15,000, preferably 2,000 to 10,000 is dissolved in a non-halogen solvent together with a crosslinking agent and a crosslinking catalyst, and a crosslinking reaction is then conducted under heating, thereby obtaining a biodegradable coating film. However, application fields thereof are limited because it is a low-molecular weight biodegradable polymer including an oligomer, and the crosslinking reaction is followed.
There is thus a demand for development of a biodegradable aliphatic polyester such as PLA or PGA, which can be dissolved in an organic solvent under an environment of a low temperature such as ordinary temperature.
On the other hand, the biodegradable aliphatic polyester such as PGA or PLA is generally stored or shipped in the form of particles of the biodegradable aliphatic polyester at the stage of distribution. Thereafter, the particles are formed or molded according to various forming or molding methods suitable for an intended formed or molded product or submitted for appointed uses through the form of a dispersion liquid or solution of the particles.
A production process of resin particles of the biodegradable aliphatic polyester such as PLA particles or PGA particles is variously proposed.
As a production process of the biodegradable aliphatic polyester particles, are generally known a production process of particles by cutting or grinding of a melted and solidified product, and a production process of particles by deposition from a solution or dispersion liquid. Japanese Patent Application Laid-Open No. 2001-288273 (Patent Literature 4) discloses a production process of polylactic acid-based resin powder, in which chips or a massive product composed of a PLA resin is refrigerated to a low temperature of −50 to −180° C., impact-ground and classified. Japanese Patent Application Laid-Open No. 2006-45542 (Patent Literature 2) mentioned above discloses that a solution of PGA or PLA is quickly cooled at a cooling rate of at least 20° C./min to obtain particles having an average primary particle diameter of 10 to 1,000 nm.
However, these processes are difficult to industrially provide particles of the biodegradable aliphatic polyester such as PGA or PLA, which are soluble in an organic solvent under a low-temperature environment, and there is thus a demand for development of a process for industrially producing particles of the biodegradable aliphatic polyester such as PGA or PLA, which are soluble in an organic solvent under an environment of a low temperature such as ordinary temperature.